Influence Of Short-range Order Research Articles

Influence of short-range order on diffusion in multiprincipal element alloys from long-time atomistic simulations

Short-range order (SRO) is an important structural characteristic in multiprincipal element alloys (MPEAs) that plays a crucial role in their superior mechanical and physical properties. The development of SRO is usually tangled with structural evolution associated with diffusion-induced atomic transport, making it challenging to accurately determine the diffusion coefficients in MPEAs based on conventional modeling techniques. In this study, we combine machine learning with kinetic Monte Carlo to compute the self-diffusion coefficients in NiCoCr and NiFeCr MPEAs with distinct SRO tendencies under long-time structural evolution. Our results show that the long-term diffusion process governed by thermodynamic and kinetic factors can promote SRO formation due to chemically biased exchange between vacancy and specific components in the alloy. We further demonstrate that accurate diffusion coefficients in MPEAs can only be obtained through long-term simulations allowing for sufficient structural evolution. This study underlines the shortcomings of the presently widely employed techniques to study diffusion in MPEAs and highlights the intricate coupling between SRO and diffusion.

Influence of group IV element on basic mechanical properties of BCC medium-entropy alloys using machine-learning potentials

In order to elucidate the origin of excellent mechanical properties of high-entropy alloys (HEA), it is essential to develop the atomic-level depiction of defect structures, taking into account the influence of composition. Especially in body-centered cubic (BCC) HEA an alteration of constituent elements may lead to a dramatic change in the behavior on the macroscopic level. To study that effect, we employed a machine learning technique and constructed highly accurate robust potentials for two BCC medium-entropy alloys: MoNbTa and ZrNbTa. Even though they have close composition, the mechanical properties of the two alloys differ not only quantitatively, but also qualitatively. We show that the group IV element Zr decreases values of bulk and elastic constants. The influence of short-range order on stacking fault and twin boundary energies is discussed. Also, we show the difference in the screw dislocation core shapes between the two alloys. The cores of 〈111〉 screw dislocations in the ZrNbTa case demonstrate a non-compact shape substantially extended on the (110) plane.

Resistivity of Cu, Ni and Sb in undercooled liquid state

With the help of non-contact measurement, electrical resistivity of Cu, Ni, Sb has been measured to study the influence of short range order on electrons scattering in the normal and undercooled liquid state. Till the maximum undercooling, linear resistivity-temperature dependence is obtained in Cu and Ni melts. Nonlinear resistivity behavior is obtained in undercooled liquid Sb till a maximum undercooling of about 60 degree. The increase in viscous flow activation energy reveals structure transition in Sb melts. A drop in resistivity of Cu melts is obtained accompanying the recalescence. The abrupt jump in resistivity of Sb reveals the rapid growth of Peierls distortion during rapid solidification process.

Tetragonality and the distribution of carbon atoms in the Fe–C martensite: Molecular-dynamics simulation

In the statistical theory of the ordering of carbon atoms in the z sublattice of martensite, the most important role is played by the parameter of the strain interaction of carbon atoms λ0, which determines the critical temperature of the bcc–bct transition. The values of this parameter (6–11 eV/atom) obtained in recent years by the methods of computer simulation differ significantly from the value λ0 = 2.73 eV/atom obtained by A. G. Khachaturyan. In this article, we calculated the value of λ0 by two methods based on the molecular-dynamics simulation of the ordering of carbon atoms in the lattice of martensite at temperatures of 500, 750, 900, and 1000 K in a wide range of carbon concentrations, which includes ccrit. No tails of ordering below ccrit have been revealed. It has been shown analytically that there is an inaccuracy in the Khachaturyan theory of ordering for the crystal in an elastic environment. After eliminating this inaccuracy, no tails of the order parameter appear; the tetragonality changes jumpwise from η = 0 to ηcrit = 0.75 at ccrit = 2.9kT/λ0 instead of ηcrit = 0.5 and ccrit= 2.77kT/λ0 for an isolated crystal. Upon the simulation, clustering of carbon atoms was revealed in the form of platelike pileups along {102} planes separated by flat regions where no carbon atoms were present. The influence of short-range order in the arrangement of neighboring carbon atoms on the thermodynamics of ordering is discussed.

Short range order fluctuations and itinerant ferromagnetism in Ni3Al

Density functional calculations, using the SIESTA code, have been performed for determining the influence of short range order, SRO, on the magnetic properties of Ni75Al25 (at%). Calculations have been performed on a supercell formed by eight cells in the fcc L12 structure that corresponds to a number 24+n of Ni atoms and 8−n of Al atoms, where n measures the deviation to the ordered L12 structure composition. Three values of n were considered, 0, 1 and −1. It turns out that the electronic density of states at the Fermi level, the magnetic moment and the susceptibility strongly depend on the value of n. Magnetic measurements and structural characterization were carried out before and after annealing particles of Ni75Al25at% composition, with size below 25μm, obtained by gas atomization process. The as atomized samples exhibit a broad distribution of Curie temperatures that reflects SRO fluctuations. After annealing the distribution width drastically decreases and noticeable modifications in the magnetic properties, as Curie temperature and magnetic moment, are observed. Since annealing keeps roughly constant the composition, these modifications should be attributed to the thermally induced increase of SRO. The influence of chemical short range order, CSRO, determined by X-ray diffraction, on the experimental magnetic properties is discussed on the light of the density functional calculations. The remarkable controversial values of the magnetic properties found in the literature can be well understood within the framework of the analysis reported below.

Growth of epitaxially oriented Ag nanoislands on air-oxidized Si(1 1 1)-(7 × 7) surfaces: Influence of short-range order on the substrate

Clean Si(111)-(7×7) surfaces, followed by air-exposure, have been investigated by reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). Fourier transforms (FTs) of STM images show the presence of short-range (7×7) order on the air-oxidized surface. Comparison with FTs of STM images from a clean Si(111)-(7×7) surface shows that only the 1/7th order spots are present on the air-oxidized surface. The oxide layer is ∼2–3nm thick, as revealed by cross-sectional transmission electron microscopy (XTEM). Growth of Ag islands on these air-oxidized Si(111)-(7×7) surfaces has been investigated by in situ RHEED and STM and ex situ XTEM and scanning electron microscopy. Ag deposition at room temperature leads to the growth of randomly oriented Ag islands while preferred orientation evolves when Ag is deposited at higher substrate temperatures. For deposition at 550°C face centered cubic Ag nanoislands grow with a predominant epitaxial orientation [1 1¯ 0]Ag||[1 1¯ 0]Si, (111)Ag||(111)Si along with its twin [1¯ 1 0]Ag||[1 1¯ 0]Si, (111)Ag||(111)Si, as observed for epitaxial growth of Ag on Si(111) surfaces. The twins are thus rotated by a 180° rotation of the Ag unit cell about the Si[111] axis. It is intriguing that Ag nanoislands follow an epitaxial relationship with the Si(111) substrate in spite of the presence of a 2–3nm thick oxide layer between Ag and Si. Apparently the short-range order on the oxide surface influences the crystallographic orientation of the Ag nanoislands.

The influence of short range order on the thermodynamics of Fe–Cr alloys

Using atomistic simulations of Fe–Cr alloys and computational thermodynamics techniques, we study the influence of short range order (SRO) on the location of the α–α′ miscibility gap. By comparing the random alloy with the short range ordered alloy, we extract the contributions of SRO to the free energy coming from the enthalpy of mixing and from the vibrational and configurational entropies. We conclude that the effects of SRO are significant, doubling the solubility limit of Cr at low temperatures (≈300 K), and that this effect is mainly due to the contribution of SRO to the enthalpy. The result is relevant to the nuclear applications of these alloys where irradiation accelerates α′ precipitation.

Local order in industrial and model γ phases of superalloys

Different γ phase model alloys of nickel base superalloys are investigated as a function of temperature using elastic diffuse neutron scattering, macroscopic tensile tests, TEM post mortem observations and in situ straining experiments. Short Range Order (SRO) of (1 1/2 0) type resulting from nano-domains with correlation lengths of 0.5–1 nm is observed in all alloys. The dislocation distribution confirms the presence of short-range ordered nano-domains with a size of about 0.7 nm. In all the alloys, the influence of SRO on the deformation mechanisms is clear at low and medium temperature and thus explains the similarities of the deformation behaviour observed in this temperature range. From 750°C to 1000°C, the deformation microstructure becomes progressively homogeneous which suggests the decrease of SRO as illustrated by a decrease of diffuse scattering intensities at the (1 1/2 0) positions on heating. The various techniques applied prove to be complementary in the investigation of complex solid solutions.

Electronic structure of random binary alloys beyond the single-site approximation

We study the influence of short-range order (SRO) and correlated scattering by pairs of atoms on the electronic structure of random binary alloys. Our approach is based on the gamma expansion method (GEM) proposed by Tokar, and implemented within a tight-binding linear-muffin-tin orbital (TB-LMTO) formulation of the (single-site) coherent potential approximation (SCPA) which approximately accounts for geometric relaxation. Results are presented for ${\mathrm{Ag}}_{0.87}{\mathrm{Al}}_{0.13},$ in which the SRO has recently been measured, and ${\mathrm{Cu}}_{0.715}{\mathrm{Pd}}_{0.285},$ for which older data exist, and where lattice relaxation effects play an important role. In both cases we find that the density of states (DOS) of the system is only weakly affected by the SRO. The applicability of the GEM to a first principles determination of the degree of SRO is briefly discussed.

The Influence of Short Range Order on the Energetics of Ni-Rich Nial Alloys

AbstractTotal energy calculations for Ni-rich ß′-phase NiAl have been performed using the large system multiple scattering (LSMS) method. The large samples used to model the alloys involved up to 128 atoms per cell, and were constructed to have the experimental short range order (SRO) parameters. Both short range ordering and charge transfer effects are automatically taken into account in the calculation. The calculated formation energies of both stoichiometric and non-stoichiometric compounds are in excellent agreement with experiment.

On the influence of short range order upon the physical properties of triphenylene nematic discogens

Abstract The dielectric and elastic properties of two novel triphenylene nematic discogens are measured and compared with those of calamitic nematics and the known material H7OBT. The new compounds have nematic phases at much lower temperatures than H7OBT. All three compounds have a small positive permittivity anisotropy, indicating the presence of a permanent dipole contribution parallel to the director. Measurement of the molecular moment dipole provides a value consistent with nearly-free rotation of the substituent groups. The temperature dependence of ∊ and the concentration dependence of the dipolar strength Δ show evidence of antiparallel dipole correlations in the ND phase. The strong temperature dependence of |k 33/Δχ|, the large value of k11/k 33 and the rise in k 11/k 33 as the temperature approaches the clearing point are believed to be due to short range columnar order. Theoretical study suggests that dipole correlations favour local columnar order, and we predict that antiparallel dipole correlations require that the molecules be tilted within the columns.

Influence of Short-Range Order on the Electron–Phonon Interaction in Disordered Binary Alloys

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Changes of the electronic structure of Cu-Pt due to order-disorder transitions.

The changes of the electronic structure of Cu-Pt undergoing an order-disorder transition have been investigated by measuring the linear coefficient of the low-temperature specific heat \ensuremath{\gamma} and the nuclear spin-lattice relaxation time ${\mathit{T}}_{1}$ of $^{195}\mathrm{Pt}$. The observed changes of the values of \ensuremath{\gamma} and ${\mathit{T}}_{1}$ can be interpreted by comparing them with theoretical values based on relativistic Korringa-Kohn-Rostoker coherent-potential-approximation and embedded-cluster-method calculations. The influence of short-range order on the electronic structure has also been investigated.

First principles calculation of thermodynamic properties of noble-metal alloys

The electronic structure calculation is incorporated in the cluster-variation method, and thermodynamic properties of disorder phase of noble-metal alloys are studied. For Cu-Au and Ag-Au systems, the heats of formation are estimated as functions of concentration, and the influence of short range order on the lattice constant is quantitatively discussed at Cu 3Au stoichiometry. For Ag-Cu system, phase diagram is obtained based on the same model.

Influence of Short Range Order, Electron Concentration and Size Misfit on the Structure and Thermal Stability of Metallic Glasses

ABSTRACTThe influence of short range order, electron concentration and size misfit on the thermal stability of (Fe100-xMx)83B17 metallic glasses has been investigated (M = Cr, Mn, Co, Ni, Mo, W). The results are discussed together with Curie-temperature measurements and Mößbauer spectroscopy data. The concentration range in which the various stable and metastable phases form is given.

Diffusion of Classical Particles in a Random Medium. II. Nonlinear Response

A formalism developed in a preceding paper [J. Phys. Soc. Jpn. 51 (1982) 1482.] is extended and applied to a nonlinear response of a single classical particle diffusing in a random array of medium particles which behave as obstructions or traps. The expressions of the mobility in second and inverse-second order in the strength of the external force are derived on the basis of a given radial distribution function of medium particles. The mobility is calculated explicitly for hard-sphere and attractive Gaussian core systems and influence of short-range order between medium particles is revealed. For hard-sphere systems, the calculations show that the strong external force causes the capture of the diffusing particle in gaps formed by medium particles.

Influence of short range order on optical phonons in PdD 0.74

A series of transverse and longitudinal optic phonons has been measured in PdD 0.74 in the short range ordered state at 65 K and in the disordered state at 110 K by inelastic neutron scattering. A small change of the frequencies of the TO phonons were found at 65K in the [ζ00] and [ζζ0] directions. A LO phonon shows a narrowing of the peak in the short range ordered state.

The influence of short-range order on electronic properties of transition metals: II. Amorphous metals

Local electronic properties have been calculated for the amorphous transition metals Nb, Mo, and Tc using a cluster model which accounts for the topology of the local structure. Trends in the density of states observed in metallic glasses are reproduced, and the behaviour of the averaged electron-phonon coupling strength correlates well with the behaviour of the superconducting transition temperature. The ratios of the critical temperatures in the amorphous and the crystalline state are reproduced correctly. The results show that the change in the critical temperature on going from the crystalline to the amorphous state can be explained by the change in short-range order.

The influence of short-range order on electronic properties of transition metals: I. Crystalline systems

Densities of states and electronic parameters related to superconductivity have been calculated for the transition metals Nb and Tc, representing the bulk materials by a finite cluster of atoms. The Schrodinger equation is solved for atomic potentials which have been obtained in a self-consistent band structure calculation. The results show that, in these systems, the electronic structure is determind predominantly by short-range order.

Short-range order in germanium-silicon alloys (effect on electronic properties)

The cluster-Bethe-lattice method and a simple model Hamiltonian have been used to determine the influence of short-range order on the valence band and direct optical gap of Ge-Si alloys. The results predict that an ionic gap occurs for only a very narrow range of concentration and correlation, and that the deviation from linearity of the direct optical gap decreases as the probability of Ge-Si bond increases. A possible method of deriving the degree of short-range order from photoemission studies is suggested.